From all the reading I have done recently about chemistry in a reef, I just want to point out how important it is to maintain magnesium levels along with Ca and alk. If there is depressed Mg in a system, Ca will not be utilized for coralline or stony coral growth, no matter how balanced alk and Ca are. Always figure Mg into your equation. I guess that is why the Reef Chemistry Calculator includes Mg along with Ca and alk:
http://home.comcast.net/~jdieck1/chem_calc3.html
Remember, too, when calculating the dimensions of your tank to use the INSIDE measurements of the tank, not the outside measurements, for the Aquarium System Volume Calculator:
http://home.comcast.net/~jdieck1/volcalc.html